1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to a structure of a fixing device contained in the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic type, a photosensitive drum is substantially uniformly charged, and then a laser scanning unit or the like conducts exposure on the photosensitive drum to form an electrostatic latent image according to an image signal. Then, toner that is charged by a developer is supplied onto the photosensitive drum to visualize a toner image, which is transferred onto a recording paper sheet such as a transfer paper sheet. The toner image transferred onto the recording paper sheet is merely born on the recording paper sheet, and is not fixed thereto. Therefore, a fixing unit arranged in the image forming apparatus applies heat and pressure for thermally welding and fixing it so that a fixed image is formed on the recording paper sheet.
In a basic structure of the conventional fixing device, a heating member containing a halogen lamp, a pressing member pressed against the heating member and a thermistor sensing a temperature of the heating member are arranged in a casing. In this fixing device, a toner image passing through a nip portion formed by pressing the heating member and the pressing member together is heated by heat of the heating member heated by radian heat of the halogen lamp, and receives a pressure caused by the heating member and the pressing member pressed together so that the toner image is fixed to a recording paper sheet. The casing is provided with an entry port for taking in the recording paper sheet and an exit port for discharging the recording paper sheet. The recording paper sheet bearing the toner image is taken into the casing through the entry port, and the recording paper sheet subjected to the fixing is discharged from the casing through the exit port.
A fixing device disclosed in Japanese Laid-Open Patent Publication No. 08-190303 is provided with a cooling device such as a fan for adjusting an inside temperature of the fixing device.
However, the structure provided with the fan does not allow reduction in size of the fixing device, and also suffers from a problem relating to efficient use of thermal energy that was used for heating the fixing device because the rotating fan forcedly cools a whole inside of the fixing device.